Health insurer Medibank Private says it has been hit by a cyber attack. 

The company said "unusual activity" had been detected on its network on Wednesday, but there was no evidence sensitive data, including customer information, had been accessed. 

Some customer-facing systems have been taken down which will cause "regrettable disruptions" to some customers, but health services will still be available, Medibank said. 

It is the latest cyber attack after the Optus breach last month, which affected millions of customers.

Chief executive David Koczkar said Medibank recognised its responsibility, given the sensitive data it holds about people's health. 

"I apologise and acknowledge that in the current environment this news may make people concerned," he said. 

"Our highest priority is resolving this matter as transparently and quickly as possible."

Medibank's ahm and international student policy management systems are currently offline.

Investigations into the cyber attack are ongoing, the company said.

Medibank requested a trading halt on the ASX while it investigated.